The display surface of an image display such as liquid crystal displays (LCD), cathode ray tube display apparatus (CRT) and the like is required to manifest little reflection of beam emitted from an outer light source such as a fluorescent lamp and the like in order to improve visibility.
A phenomenon is conventionally known in which reflection factor decreases by coating the surface of a transparent body with a transparent layer having small reflection factor, and it is possible to improve visibility by providing an antireflection coating utilizing such a phenomenon on the display surface of an image display. The antireflection coating has a layer structure in which, for the purpose of securing sufficient hardness, a hard coat layer is provided on a substrate, and on this is provided a low refractive index layer having smaller refractive index than that of the hard coat layer, to make the refractive index of the outermost surface smaller, or a layer structure in which, for further improving the antireflection effect, one or more middle to high refractive index layers are provided on the above-mentioned hard coat layer, and a low refractive index layer is provided on the middle to high refractive index layer, to make the refractive index of the outermost surface smaller.
A high refractive index layer or middle refractive index layer in such an antireflection coating are required to have optical properties such as refractive index, transparency and the like by which a sufficient effect can be manifested when made into an antireflection coating, and physical properties such as adhesive property with adjacent other layers (hard coat layer, low refractive index layer), scratch resistance and the like.
Methods of forming a high refractive index layer or middle refractive index layer in an antireflection coating are in general classified roughly into gas phase methods and application methods, and the gas phase methods include physical methods such as a vacuum deposition method, sputtering method and the like, and chemical methods such as a CVD method and the like, and the application methods include a roll coat method, gravure coat method, slide coat method, spray method, immersion method and screen printing method, and the like.
In the case of the gas phase method, it is possible to form a high refractive index layer and a middle refractive index layer having high functions and high quality, however, there are problems that precise control of atmosphere in a high vacuum system is necessary, a special heating apparatus or ion generating-accelerating apparatus is necessary, and for this reason, a production apparatus is complicated and its size increases, leading necessarily to increase in production cost. Further, it is difficult to enlarge the area of a thin layer of a high refractive index layer and middle refractive index layer or to form a thin layer having uniform thickness on the surface of a film having complicated form.
On the other hand, in the case of the spray method among application methods, there are problems that an efficiency to utilize a coating liquid is poor, and control of layer forming conditions is difficult, and the like. In the case of a roll coat method, gravure coat method, slide coat method, immersion method, screen printing method and the like, an efficiency to utilize layer formation raw materials is good, and merits are present in large scale production and equipment cost, however, there is generally a problem that a high refractive index layer and middle refractive index layer obtained by an application method are poor in functions and quality in comparison with those obtained by a gas phase method.
Recently, as an application method capable of forming a thin layer of a high refractive index layer and a middle refractive index layer having excellent quality, a method is suggested in which a coating liquid prepared by dispersing fine particles with high refractive index such as titanium oxide, tin oxide and the like in a solution of a binder composed of an organic substance is applied on a substrate, to form a coating layer.
Since it is essential that a coating layer forming a middle to high refractive index layer is transparent in the visible light region, it is necessary that, as high refractive index fine particles, so-called ultra-fine particles having a primary particle size of not larger than the wavelength of visible beam are used, and the above-mentioned high refractive index fine particles are uniformly dispersed in a coating liquid and a coating layer. However, in general, when the particle size of fine particles decreases, the surface area of fine particles increases, and aggregation force between particles increases. When solid components of a coating liquid aggregate, haze of the resulting coating layer deteriorates. Accordingly, a coating liquid for forming a thin layer of a high refractive index layer and a middle refractive index layer is required to have sufficient dispersibility to form an uniform coating layer showing small haze. Further, the coating liquid is required to have sufficient dispersion stability so that it can be stored easily for a long period of time.
A problem of aggregation of ultra-fine particles can be solved by use of a dispersing agent showing excellent dispersibility for the ultra-fine particles. A dispersing agent adsorbs on the surface of a fine particle while permeating between fine particles to be aggregated, and enables uniform dispersion of fine particles in a solvent while relaxing aggregated state in a process of dispersion treatment. However, since ultra-fine particles have increases surface area, a large amount of dispersing agent is necessary for dispersing ultra-fine particles uniformly in a coating liquid and for stabilizing ultra-fine particles so that they can stand storage for a long period of time. When a large amount dispersing agent is mixed in a coating liquid, the dispersing agent is present in large amount also in a coating layer formed using the above-mentioned coating liquid, and the dispersing agent prevents curing of binder components, and significantly lowers the strength of the coating layer.
Further, a coating liquid is required to have such application aptitude that it can be applied uniformly and thinly so as to easily form a thin layer with large area from the standpoint of mass production, and drying unevenness does not occur.
The middle to high refractive index layer is required to have sufficient adhesive property with a hard coat layer and a low refractive index layer respectively adjacent to the middle to high refractive index layer. When a low refractive index layer such as a silicon oxide (SiOx) layer and the like is formed by a so-called dry coating method such as a vapor deposition method and the like on a middle to high refractive index layer formed from a coating liquid by a so-called wet method, adhesive property is extremely deficient and releasing occurs easily, therefore, particularly excellent adhesive property is required.
The hard coat layer has originally plays a role as a supporting layer for a middle to high refractive index layer in order to for prevent scratching of an antireflection coating, and in the case of obtaining a high refractive index hard coat layer having simultaneously a function as a middle to high refractive index layer by compounding high refractive index fine particles in this hard coat layer, the constituent layer number of an antireflection coating can be reduced. However, while the middle to high refractive index layer has a thickness of from about 5 to 200 nm, preferably from about 50 to 160 nm, the hard coat layer is formed so as to have fairly large thickness of from about 0.2 to 20 μm, preferably from about 1 to 10 μm, further preferably from about 2 to 5 μm for the original purpose of securing sufficient hardness, therefore, when a high refractive index hard coat layer is formed by a wet method using the same coating liquid as that for the middle to high refractive index layer, there is a tendency of causing deterioration in transparency due to aggregation of high refractive index fine particles further than in the case of forming a middle to high refractive index layer by a wet method. Additionally, though the hard coat layer is required to have high hardness, a dispersing agent has a nature of preventing curing of a binder of a coating layer as described above, therefore, the amount of a dispersing agent which can be mixed in a coating liquid for a hard coat layer is limited further than in the case of a coating liquid for a middle to high refractive index layer. Consequently, a requirement for reduction of a dispersing agent in a coating liquid for a high refractive index hard coat layer is further severer than that for a coating liquid for a middle to high refractive index layer.